An ignition system for a gas turbine engine typically includes an electrical power source, two ignitor plugs, separate exciters for each ignitor plug and the associated cables and wire harnesses. Each exciter converts either an AC or DC low voltage value to a high voltage value for delivery to the exciter's associated ignitor plug. Dual ignitors are used in an aircraft gas turbine engine to ensure the failure of a single ignitor will not result in loss of the ability to light or re-light an engine.
Ignition systems are not only used for engine starting, but also for ignition stand-by protection to relight the engine in the event of an in-flight flameout while operating under potentially unstable flight conditions such as icing, air turbulence, takeoffs, low approaches, go-arounds and landings. A small change in airflow at the compressor inlet, or at the entrance to the aircraft inlet duct, may cause a condition for which the fuel control can not immediately compensate, and a flameout results. Such a condition may occur by flying in turbulent air, or perhaps, by bird ingestion or ingestion of ice broken off the engine inlet. If a flameout does occur while one or more of the exciters are operating, the engine should relight automatically as soon as fuel control compensation takes place, and the abnormal inlet condition corrects itself. Ideally, the time from flameout to automatic relight is so fast the brief flameout should be transparent to the pilot.
Although aircraft gas turbine engines can be ignited quite easily under ideal situations, aircraft gas turbine engines typically operate at high altitudes (e.g., 36,000 feet) where conditions for an engine relight in the event of a flameout are less than ideal. The low temperatures encountered at high altitude cause a decrease in fuel volatility which contributes to the difficulty of re-igniting the fuel. While it may be advantageous to operate the ignitors continuously from a safety point of view for an automatic relight, continuous operation significantly reduces the ignitor's operational life due to the high pulsed voltage operation of the ignitor.
A well known type of pulsed voltage ignition system is the high energy capacitor ignition system which employs a DC capacitive discharge arrangement to apply the high pulsed instantaneous voltage to the ignitor. For an aircraft gas turbine engine, the exciter typically uses a DC-to-DC converter circuit which converts the conventional 28 VDC aircraft power bus signal to a high voltage which is used to charge a storage capacitor. The storage capacitor is discharged into the ignitor plug which results in a very short duration (typically less than 100 microseconds) high temperature (e.g., approx. 10,000.degree. C.) plasma of ionized air across the ignitor gap. The high temperature plasma of ionized air ignites the fuel in the vicinity of the ignitor gap to initiate combustion. Due to the rapid temperature increase of the pulsed plasma, a high pressure shock wave occurs which results in the local fatigue of the ignitor and erosion of ignitor components (e.g., the electrodes and the insulator material).